1. Field of the Invention
The present invention relates to a radio LAN (Local Area Network) system and a communication method therefor.
2. Description of the Background Art
The radio LAN system is advantageously applicable to private premises in which an access point forms a service area and communicates with stations lying in the service area by radio. The communication method is advantageously applicable to a multimedia communication procedure.
A radio LAN system constructed in private premises has customarily used a system prescribed by IEEE (Institute of Electrical and Electronics Engineers) 802.11b. This system is unique in that a direct spread spectrum scheme is applied to a radio system and in that a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) scheme is applied to a multiplexing system. The CSMA/CA scheme is desirable because transmitted power in a radio LAN system is generally far greater than received power and cancels received power to thereby obstruct collision detection.
The CSMA/CA scheme has two different access systems, i.e., a DCF (Distributed Coordination Function) access system and a PCF (Point Coordination Function) access system. The DCF access system uses the concept of LAN and does not need an access point, which the PCF access system needs. Today, the direct spread spectrum system is applied to cellular phones because it is highly resistive to noise and interference and desirable in secrecy.
On the other hand, an access point or radio LAN base station and mobile stations are sometimes constructed into a single radio LAN system. This kind of radio LAN system has a single spread code and therefore uses a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) scheme as a multiplexing system. The CSMA/CD scheme causes any one of the stations to make an access after determining whether or not another station is emitting a radio wave. Therefore, a transmission time includes a collision detection time.
The CSMA/CD system has the following problems left unsolved. The collision detection time, among others, limits the effective transmission rate in a single system to only about 70% of a transmission rata available with a communication path, as known in the art. In addition, additional information contained in overheads reduces the amount of information to be sent.
Assume that different types of information, e.g., real-time voice information and usual file data exist together. Then, the CSMA/CD system transmits the different types of information by the same procedure. Consequently, voice information, needing real-time communication, is sometimes forced to wait until the end of transmission of file data. More specifically, voice information is apt to be delayed and therefore discontinuous in speech.
Further, the CSMA/CD system lacks an error correcting function although having an error detecting function in the communication procedure. It is therefore a common practice with the CSMA/CD system to assign the correction of an error having occurred in a radio section or interface to an upper-layer communication procedure. Usually, the error correction is implemented by a re-transmitting function available with an upper-layer communication procedure. However, delay ascribable to re-transmitting is not allowable when it comes to voice information that must be real-time. Consequently, interruption of voice information frequently occurs e.g. in private premises in which a communication path is often disconnected or the strength of a radio wave varies due to the movement of a speaker.
The only way available for reducing the interruption of voice information is decreasing the service area of each access point and guaranteeing received field strength or increasing the output power of each terminal. However, increasing the output power of each terminal brings about interference with another terminal.
As stated above, even when the CSMA/CD scheme is applied to a radio LAN system, it is difficult to realize simultaneous transmission of voice information and file data with the LAN system due to the delay particular to the CSMA/CD scheme and errors ascribable to the varying radio wave environment in a radio section.